1. Field of the Invention
The present invention relates to a magnetostrictive torque sensor which detects torque based on a change in magnetic characteristics resulting from a magnetostriction. The magnetostrictive torque sensor includes a shaft having a magnetostrictive film provided thereon, coils which surround the shaft, and a torque detecting device which detects a change in magnetic characteristics that occur when a rotational torque is applied to the shaft.
2. Description of Related Art
JP2001-13337 discloses a steering torque sensor of an electric power steering apparatus which detects a steering torque applied to a steering wheel. The steering torque sensor detects the steering torque using the measured inductance variation of the coils which results from a change of the permeability of the magnetostrictive film that corresponds to a rotational torque of the shaft.
When the shaft of a steering torque sensor is magnetized, a magnetic characteristic of the shaft changes and the accuracy in torque detection decreases. Therefore, there is a need to erase the magnetization of the shaft. Magnetostriction is a well-known property of ferromagnetic materials that changes the shape of the materials due to stress or torque, wherein a magnetic field is changed. An example of such a magnetic material is Ni—Fe. The magnetostriction of the shaft is known to be erased by providing an alternate current provided to magnetic erasing coils surrounding the shaft. However, since the steering torque detecting coils are already provided near or on the shaft, it is difficult to also arrange the magnetic erasing coils and the torque detecting coils on the shaft. Even if the magnetic erasing coils and the steering torque detecting coils are arranged, respectively, on the shaft, the overall size of the magnetostrictive torque sensor will undesirably increase and the overall construction of the torque sensor will become complex.